MPCe/LPCe Users Manual
31
Field
Size
Comment
1. START character
1 character (byte)
ASCII character is ‘~’ (TILDA)
Start is the first byte in the command packet and tells remote controllers to start decoding a message. It should
be implemented as a #define, so that it can be changed if necessary. As a #define, the character is rarely
changed because it is hard coded into the MPC.
<SPACE>
2. ADDRESS
2 hex characters
Range 00 through FF
This field should be filled in with the hexadecimal representation of the integer address of the controller. The
range provides 255 unique addresses. Only 32 devices may reside on the same serial port due to hardware
loading limitations.
<SPACE>
3. COMMAND CODE
2 hex characters
Range 00 through FF
This field is one of 255 possible hexadecimal numbers, which is typically an index into a table of functions on the
remote controller. Commands should be implemented as #defines with integers between 0 and 255. The integer
value must be converted into ASCII hex before placement into the command packet character array. The
command code must be two hex digits, even if the first is a zero.
<SPACE>
4. DATA field(s), optional
as needed
ASCII printable characters only
Data field(s) are for any commands that have a data value. For instance, a command to set a beam voltage in
some unit would probably consist of a command to set the beam voltage, along with a value of beam voltage. If
the command had more than one data value associated with it, such as setting an X and a Y value in a unit, the
command field could be followed by two data fields (X and Y) separated by a space between them. All data must
be sent in ASCII printable format (no binary or “control” characters). There is no limit on the number or size of
data fields. It is up to the remote unit designer to keep practicality in mind when determining these fields. A data
field is not required for all commands.
<SPACE>
5. CHECKSUM
2 hex characters
Computed checksum of packet
The calculated checksum must have its value in ASCII hexadecimal notation. It is calculated by adding the
decimal value of all characters in the packet (excluding start, checksum, and terminator), and then dividing the
result by 256. The integer remainder converted to two ASCII hex digits is the checksum. When a remote device
receives a packet, the passed checksum is compared with a computed checksum and if they do not match, the
device discards the packet.
6. Terminator character
2 hex characters
ASCII carriage return
This field is an ASCII carriage return placed at the end of a command packet. This character is not the newline
character “\n”, but can be used in string assign statements as “\r”. There is not a space between the checksum
and terminator field.
Decoding the Command Packet
A MPCe/LPCe operates in one of three modes. Receipt of data is interrupt driven or otherwise multiplexed so
that other tasks are performed by the unit's microprocessor. When the controller receives a command packet, it
continues monitoring for new commands while the current one is carried out.
1. MONITOR. The controller monitors serial data traffic. When a "start" character is detected, the controller
changes to the RECEIVE MODE.
2. RECEIVE. After receiving start, the controller tests subsequent characters for a valid command packet. This
mode must have a count down timer associated with it, which is a predetermined time for a valid command packet
to be received. Furthermore if another start character is received while in this mode, (i.e. the first start character
was actually part of a command packet for a different device) the controller responds by going back to the
beginning of the RECEIVE mode, with a fresh timer setting. If a command packet is not received within the
